The annual maintenance and recurring costs are almost negligible, since there are no moving parts and the input fuel (sunlight) is free. For optimum performance, the system only requires cleaning of modules and basic preventive and corrective maintenance. However, for off-grid systems where batteries are used, the maintenance costs are higher on account of battery replacement every 3-5 years. To ensure high generation and low maintenance cost, regular monitoring through data loggers is highly recommended. Typically, the maintenance costs for smaller Solar PV systems is about 2% of the initial system cost, and for larger systems is about 1% of the initial cost.